Jan. 22, 2013 — Regular aspirin users are more likely to develop the “wet” form of age-related macular degeneration compared to people who rarely or never take the drug, a new study shows.

Aspirin is one of the most widely used drugs in the world. Millions of people with heart disease take a daily low dose of aspirin in hopes of preventing heart attacks and stroke. It’s also used to ease pain.

Macular degeneration is a leading cause of blindness in older adults, and it is on the rise. The “wet” form accounts for only about 10% to 15% of cases, but it progresses more rapidly and is more likely to lead to vision loss than the “dry” form.

In “wet” macular degeneration, tiny new blood vessels grow under the retina, the light-sensing part of the eye. These blood vessels break open and leak, causing scar tissue to form. Over time, the scar tissue clouds central vision. It’s not clear why this happens.

Both kinds of macular degeneration become more common as people age. Beyond age, the only risk factor that’s consistently been linked to the condition is smoking.

News that aspirin may be linked to macular degeneration surfaced last year when a large European study found that regular aspirin users were more likely to develop the sight-stealing disease. Before that, two large studies found no association between aspirin and macular degeneration. Another study had even suggested that aspirin might protect against the “dry” form of the disease.

Aspirin and Macular Degeneration

For the new study, researchers in Australia followed more than 2,000 older adults. Doctors conducted detailed interviews at the start of the study, asking people about a variety of diet and lifestyle habits, including medication use. About 11% of people (257) were regular aspirin users, meaning they’d taken the drug at least once a week in the past year.

Study participants had regular eye exams to check for changes to their retinas.

Fifteen years later, 63 people in the study — 15 regular aspirin users and 48 who rarely or never took it — had developed “wet” macular degeneration.

Compared to people who never took aspirin, regular users were more than twice as likely to develop macular degeneration. That was true even after researchers accounted for other things known to influence a person’s risk for macular degeneration, including age, sex, smoking, heart disease, BMI, and high blood pressure.

The findings are published in the journal JAMA Internal Medicine.

Study Strengths and Limitations

The study doesn’t prove that aspirin causes macular degeneration. Different kinds of studies are needed to understand whether aspirin may directly harm the eye.

But one theory is that aspirin ramps up a part of the immune system called the complement system. Many people with macular degeneration carry a form of a gene that keeps them from being able to turn down the complement system when needed. Researchers say the result is that the immune system may be chronically overstimulated, causing damage to the back of the eye.

Indeed, when researchers looked just at people with the risk-conferring form of this gene, the association between macular degeneration and aspirin was even stronger. It was more than four times as high for regular vs. non-regular users, suggesting that there might be a biological basis for the association.

But other limitations make the findings less reliable. Researchers only asked about aspirin use once, for example, at the start of the study, so people who stopped taking aspirin might have been misclassified. And only half the people who started the study were followed for the full 15 years; so it’s possible that people who were motivated to stick with the research may have been more concerned about their health and eyesight in general. It’s also possible that there were other differences between the groups that the study authors weren’t able to account for.

Advice for Aspirin Users

For all those reasons, many researchers and independent experts agree that no one should stop taking aspirin as directed by their doctor because of this study.

“Take your aspirin,” says Mark Fromer, MD, an ophthalmologist at Lenox Hill Hospital in New York City who was not involved in the research.

The risk of getting macular degeneration after 15 years of regular aspirin use was still relatively slight. “Obviously, your general health comes first,” Fromer says.

What does make sense, he says, is to get regular eye exams.

“Anyone who is taking aspirin and is of the age where they could be at risk, they’re the people who want to have their retinas checked,” he says.

The study researchers agree that regular eye exams are important, particularly if a close relative also has macular degeneration (AMD).

“If patients have a strong family history of AMD or have late-stage AMD already affecting one eye, physicians could raise the awareness in patients and their families about the possibility of this possible adverse effect following long-term use of aspirin,” says researcher Jie Jin Wang, PhD, a senior research fellow in the Centre for Vision Research at the University of Sydney in Australia.